1. Field of the Invention
The present invention relates to a laser apparatus using a semiconductor laser element.
2. Description of the Related Art
Conventionally, narrow-stripe single-transverse-mode semiconductor laser devices, which emit high quality laser beams, have a drawback that the practical output power is at most about 200 to 300 mW. There are two causes for this drawback. The first cause is so-called spatial hole burning. That is, since the rate of carrier supply for generating laser light is limited by the carrier diffusion process when the output power is high, carrier density decreases in a region where the laser beam intensity is high. Due to spatial hole burning, the refractive index of the semiconductor medium increases, and the waveguide mode is affected by the increase in the refractive index. Thus, the quality of the laser beams deteriorates, and kinks are produced in the current-light output characteristics. The second cause for the above drawback is the great optical output density. For example, in the case where the stripe width is 4 xcexcm, and the equivalent beam diameter in the direction perpendicular to the junction is 0.5 mm, the optical output density reaches 15 MW/cm2 when the output power is 300 mW. Therefore, heavy load is imposed on the semiconductor medium, and various characteristics of the semiconductor laser device deteriorate. Thus, it is difficult to obtain a reliable narrow-stripe single-transverse-mode semiconductor laser device which emits laser light with high output power.
In order to remove the above first cause, attempts have been made to optimize waveguide structure. In addition, in order to remove the above second cause, protection coating at end surfaces has been optimized, and window structures at end surfaces have been developed. However, these techniques are approaching their limits. Therefore, in order to realize a narrow-stripe single-transverse-mode semiconductor laser device which emits laser light with high output power, it is necessary to develop a new mode control technique, and reduce the optical density by increasing the light emission area.
Various attempts have been made to realize a semiconductor laser device which emits spatially coherent laser light, and has output power of hundreds of watts or more. For example, Botez and Schifres, xe2x80x9cDiode Laser Arrays,xe2x80x9d Cambridge Press, 1994 discloses a monolithic structure realizing a high quality laser beam, and a process for producing the structure. However, the structure and process are complicated.
In order to remedy the drawbacks of the conventional current-injection type semiconductor laser devices, U.S. Pat. Nos. 5,461,637 and 5,627,853 propose surface-emitting semiconductor laser devices which are excited with light. However, since these semiconductor laser devices utilize the thermal lens effect, i.e., the effect of increasing refractive indexes with temperature, the temperature must be raised. In addition, the above semiconductor laser devices are sensitive to temperature distribution, and the spatial oscillation mode is unstable. The spatial mode becomes further unstable when output power is high, since a cavity is generated in a carrier distribution due to generation of laser light having high output power (i.e., spatial hole burning occurs), and refractive indexes decrease with increase in the number of carriers due to the so-called plasma effect.
In addition, Nakamura et al., xe2x80x9cInGaN/GaN/AlGaN-Based Laser Diodes Grown on GaAs substrates with a Fundamental Transverse Modexe2x80x9d, Japanese Journal of Applied Physics Part 2 Letters, vol. 37, 1998, pp. L1020 discloses an InGaN-based short-wavelength semiconductor laser device. However, in this semiconductor laser device, it is difficult to emit laser light with high output power in a single transverse mode.
Further, B. Pezeshki et al., xe2x80x9c400-mW Single-Frequency 660-nm Semiconductor Laser,xe2x80x9d IEEE Photonics technology Letters, vol. 11, pp. 791, 1999 discloses an AlGaInP red semiconductor laser device. However, higher quality and higher output power are also required in this semiconductor laser device.
As described above, it is very difficult to achieve a single transverse mode oscillation with high output power in the conventional semiconductor laser devices.
On the other hand, in the conventional semiconductor-laser excited solid-state laser apparatuses, it is difficult to achieve high speed modulation of laser light by directly modulating semiconductor laser elements which are provided as excitation light sources since the lifetimes of fluorescence emitted from rare earth elements which constitute solid-state laser crystals are very long.
An object of the present invention is to provide a reliable laser apparatus which uses a semiconductor laser element, oscillates in a fundamental mode with high output power, and enables high speed modulation of output laser light.
According to the present invention, there is provided a laser apparatus comprising: a semiconductor laser element which emits first laser light having a first wavelength; a surface-emitting semiconductor element which is excited with said first laser light, emits second laser light having a second wavelength which is longer than said first wavelength, and has a first active layer and a first mirror arranged on one side of the first active layer; and a second mirror arranged outside the surface-emitting semiconductor element so that the first and second mirrors form a resonator in which the second laser light resonates. The surface-emitting semiconductor element includes a structure for controlling a spatial mode of the second laser light.
Since the output of the laser apparatus according to the present invention is obtained from the surface-emitting semiconductor element which is excited with laser light emitted from a semiconductor laser element, and the structure for controlling a spatial mode of the second laser light semiconductor laser element is provided, instability of the spatial mode due to the thermal lens effect or the plasma effect can be effectively suppressed. Therefore, it is possible to stably maintain a fundamental transverse mode in a wide output range from low power to high power (in particular, in the high output power region), and obtain high-quality laser light in the fundamental transverse mode.
Preferably, the laser apparatuses according to the present invention may also have one or any possible combination of the following additional features (i) to (xii).
(i) The above structure may have a size which is 0.1 to 10 times as large as a diameter to which the second laser light spreads at a position of the structure for controlling the spatial mode of the second laser light. In this case, the characteristics and beam shape of the output of the laser apparatus are particularly improved.
(ii) The structure may be realized by a pinhole spatial filter being arranged at a light exit end surface of the surface-emitting semiconductor element, having a pinhole, and allowing passage of the second laser light emitted by the surface-emitting semiconductor element, through only the pinhole. In this case, it is possible to increase resonator loss in high-order modes than that in the fundamental mode. Therefore, the oscillation in the high-order modes can be effectively suppressed, and resultantly oscillation in the fundamental transverse mode is realized.
(iii) In the above feature (ii), the pinhole may have a size which is 0.1 to 10 times as large as a diameter to which the second laser light spreads at a position of the structure.
(iv) The second mirror may have a limited area, be arranged in parallel with a light exit end surface of the surface-emitting semiconductor element, and realize the above structure for controlling the spatial mode of the second laser light. Since the laser light in the fundamental transverse mode can be selectively reflected by the second mirror, it is possible to increase resonator loss in high-order modes more than that in the fundamental mode. Therefore, the oscillation in the high-order modes can be effectively suppressed, and resultantly oscillation in the fundamental transverse mode is realized.
(v) In the above feature (iv), the mirror may have a size which is 0.1 to 10 times as large as a diameter to which the second laser light spreads at a position of the structure.
(vi) The first active layer may be formed in only a limited area in a plane parallel to a light exit end surface of the surface-emitting semiconductor element, and realize the structure for controlling the spatial mode of the second laser light.
(vii) In the above feature (vi), the limited area may have a size which is 0.1 to 10 times as large as a diameter to which the second laser light spreads at a position of the structure for controlling the spatial mode of the second laser light.
(viii) The laser apparatus according to the present invention may further comprise wavelength selection means arranged in the resonator.
(ix) The laser apparatus according to the present invention may further comprise polarization control means arranged in the resonator.
(x) The semiconductor laser element may have a second active layer made of an Inv1Ga1xe2x88x92v1N material, and the first active layer may be made of an Inv2Ga1xe2x88x92v2N material, where 0 less than v1 less than v2 less than 1.
(xi) The semiconductor laser element may have a second active layer made of an InGaN material, and the first active layer may be made of an AlGaInP or GaInP material.
(xii) The semiconductor laser element may have a second active layer made of an Inw1Ga1xe2x88x92w1As material, and the first active layer may be made of an Inw2Ga1xe2x88x92w2As material, where 0 less than w1 less than w2 less than 1.
In the above items (x) to (xii), xe2x80x9ca XY materialxe2x80x9d means a material which contains at least the elements X and Y when each of X and Y is a symbol of an element.